1. Field of the Invention
This invention relates to an apparatus for recording a video signal of a wide frequency band.
2. Description of the Related Art
Recently, there has been proposed a TV (television) signal of a so-called extended definition system which has a wider frequency band than the conventional TV signal of the NTSC color system and is interchangeable with the latter. Hereinafter, the extended definition system will be called the ED system and the TV signal of the ED system an EDTV signal.
An example of the signal form in which the EDTV signal is to be used for broadcasting is as follows: Hereinafter, the EDTV signal in that signal form will be called an encoded EDTV signal. FIG. 1 of the accompanying drawings shows by way of example the arrangement of a system for transmitting the encoded EDTV signal. FIGS. 2(A) and 2(B) show the frequency bands of the various components of the encoded EDTV signal. FIG. 3 shows the frequency allocation of the encoded EDTV signal to be actually transmitted. In this instance, the horizontal and vertical scanning frequencies are assumed to be the same as those of the TV signal of the NTSC system.
Referring to FIG. 1, an input terminal 1 receives a luminance signal Y having a frequency band of 6.3 MHz which is a base band as shown in FIG. 2(A). Meanwhile, color difference signals I and Q have bands of 1.5 MHz and 0.5 MHz as shown in FIG. 2(B). They are supplied to other input terminals 2 and 3 respectively.
The two color difference signals I and Q are supplied to a quadrature two-phase modulation circuit 5. They are quadrature two-phase modulated with a color subcarrier reference signal of a frequency fsc which is supplied via another input terminal 4 and a signal which is obtained by phase shifting the reference signal through a 90.degree. phase shift circuit 6. This process is carried out in a well known manner to obtain a carrier chrominance signal C. The side-bands on both sides of the chrominance signal C are respectively limited to be about 0.5 MHz by means of a band-pass filter 7 (hereinafter referred to as BPF). After the BPF 7, the signal C is supplied to an adder 15.
Meanwhile, the luminance signal Y is supplied to a high-pass filter 8 (hereinafter referred to as HPF) which is arranged to pass only a signal component of a frequency at least 4.2 MHz. The high band component YH of the luminance signal Y is thus extracted by the HPF 8 and is supplied to a subtracter 9. As a result, it is only a low-band component YL of the luminance signal Y that is obtained from the subtracter 9.
The color subcarrier reference signal is supplied to a coefficient circuit 10 to have its amplitude multiplied, for example, by 0.6. The signal thus produced from the circuit 10 and another signal which is obtained by inverting this signal at a phase inverter 11 are alternately supplied to a multiplier 13 every one-field period via a switch SW. A terminal 12 is arranged to receive a rectangular wave signal which is inverted every one-field period. The switch SW operates under the control of this rectangular wave signal.
The multiplier 13 is arranged to form a carrier high band luminance signal YH' by processing the high band luminance signal YH with a carrier .mu.0 the phase of which is inverted for every one field relative to a color subcarrier. The carrier high band luminance signal YH' has a spectrum which is shifted 30 Hz from a chrominance signal C as will be described later. The signal YH' is supplied to a low-pass filter 14 (hereinafter referred to as LPF) to have its band limited to a band not exceeding 4.2 MHz. The signal YH' is supplied to the adder 15 to be mixed with the chrominance signal C. A mixed signal thus obtained by the adder 15 is supplied via a second time-space filter 17 to an adder 18. The low band luminance signal YL is also supplied to the adder 18 via a first time-space filter 16. The mixed signal from the adder 15 and the signal YL are mixed together. As a result, the adder 18 produces an encoded EDTV signal which has frequency allocation as shown in FIG. 3 and is produced from a terminal 19 to a transmission line.
The spectral arrangement of the encoded EDTV signal is as described below with reference to FIGS. 4(A) to 4(C) and 5(A) to 5(D):
FIGS. 4(A) to 4(C) one-dimensionally show the spectrum distribution of the encoded EDTV signal. FIGS. 5(A) to 5(D) three-dimensionally show it. As shown in FIG. 4(A), the above stated signal YL and the mixed signal C + YH' are frequency interleaved with each other relative to horizontal scanning frequency fH. This is because the relation of the color subcarrier frequency fsc to the frequency fH is fsc = 1/2 fH (2n - 1), wherein n is a natural number. FIG. 4(B) shows the signals C and YL of FIG. 4(A) in an enlarged state. The relation of the horizontal scanning frequency fH to a frame frequency fF (= 30 Hz) is fH = fF (2m - 1), wherein m is a natural number. Hence, there obtains a relation fsc = 1/2 fF (2i - 1), wherein i is a natural number. In the spectrum, therefore, the positions of the signal YL and the signal C are shifted by 1/2 fF from each other. Accordingly, a vertical scanning frequency fV (= 60 Hz) is also in a frequency interleaved state. Meanwhile, the signal C has a correlativity in the temporal direction at every one-field period. Therefore, the spectrum of the signal C shows up at every 60 Hz with the peak of the spectrum located in the middle part within every frequency region for the horizontal scanning frequency fH. Assuming that the spectra of the signal C which are thus aligned at intervals of 60 Hz with their peaks located within the adjoining regions for frequency fH never infringe on each other, the spectra of the signal C are allocated within every other 30 Hz frequency region between the spectra of the signal YL which are aligned at intervals of 30 Hz as shown in FIG. 4(B). In other words, the spectrum region of 30 Hz in which the spectrum of the signal C is not allocated has heretofore been left blank. Whereas, the above stated signal YH' is allocated in this vacant 30 Hz spectrum region, as shown in FIG. 4(C).
The above stated spectrum allocation is three-dimensionally shown in FIG. 5(A) including only the signals C and YH'. Assuming a three-dimensional form including the signals YH and YL, FIG. 5(B) is a front view showing it as viewed in the time-base frequency direction. FIG. 5(C) is a sectional view taken on a plane X having a horizontal frequency x of the assumed three-dimensional form. FIG. 5(D) is another sectional view taken on a plane Y having a horizontal frequency y of the assumed three-dimensional form. In FIGS. 5(A) to 5(D), a reference symbol .mu. denotes a frequency in the horizontal direction of the image plane; a symbol .nu. denotes a frequency in the direction perpendicular to the image plane; and a symbol f denotes a frequency in the time-base direction.
Therefore, the filtering area of the first time-space filter 16 becomes as indicated by a hatched part in FIG. 6(A). In FIG. 6(A), the axis of ordinate shows the frequency in the direction perpendicular to the image plane; and the axis of abscissa the frequency in the time-base direction. The filtering area of the second time-space filter 17 becomes as indicated by hatched parts in FIG. 6(B). As well known, these time-space filters are formed by a one-horizontal scanning period delay line or by a one-frame delay device. The one-dimensional (horizontal) frequency of the encoded EDTV signal has a band-width including the signal YL between 0 and 4.2 MHz, the signal YH' between 2.1 and 4.2 MHz and the signal C of the band-width 1 MHz around 3.58 MHz.
FIG. 7 shows the arrangement of a receiving system for receiving the above stated encoded EDTV signal and to process it back into the original components signals. The encoded EDTV signal comes through the transmission line and is received at a terminal 20. The encoded EDTV signal is then supplied to a second time-space filter 21 which is arranged similarly to the second filter 17 of FIG. 1 and has its filtering area as indicated by hatched parts in FIG. 8(A). Then filter 21 then separates a component C + YH'. This component C + YH' is then subtracted from the encoded EDTV signal at a subtracter 22 to obtain thereby the component YL. The component C + YH' is further supplied to a third time-space filter 23 which has its filtering area as indicated by hatched parts in FIG. 8(B). The filter 23 separates only the component C. The component C is further separated from the component C + YH' at a subtracter 24 to obtain thereby the component YH'.
A carrier chrominance signal C is obtained through the above stated process. The signal C is supplied to a quadrature two-phase demodulation circuit 25 to be decoded there with a decoding reference signal of frequency fsc generated by a circuit 26 and a signal which is obtained by phase shifting this reference signal 90 degrees by means of a 90.degree. phase shift circuit 27. The circuit 25 thus produces the above stated two color difference signals I and Q. Meanwhile, the carrier high-band luminance signal YH' is supplied to a multiplier 31 and is converted into the original high-band luminance signal YH. The signal to be used for the multiplying operation of the multiplier 31 is a signal which is produced from the circuit 26, a coefficient circuit 28, a phase inverter 29 and a switch SW' and is phase inverted for every field. The signal YH which is obtained from the multiplier 31 is supplied to an HPF 32 to have a component below 4.2 MHz removed therefrom. After that, the signal YH is mixed with the signal YL at an adder 33. Then, the wide-band luminance signal Y is reproduced through this process.
In the case of a VTR which is arranged to record and reproduce the above stated EDTV signal, if it is possible to limit the band of the recording signal to about the same degree as the VTR used for the generally employed (ordinary) TV signal, the EDTV signal can be recorded and reproduced without increasing the relative speed between the head and the recording medium. Then, the EDTV signal can be handled by the same mechanical arrangement as that of the VTR designed for the ordinary TV signal. In that event, the ordinary TV signal also can be recorded and reproduced by one and the same apparatus as well as the EDTV signal.